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[SANET-MG] Parasitic Fungi and Pesticides Act Synergistically to Kill Honeybees?
ISIS Press Release 07/06/07
Parasitic Fungi and Pesticides Act Synergistically to Kill Honeybees?
Prof. Joe Cummins presents evidence that parasitic fungi can kill
insects when low, otherwise non-lethal concentrations of pesticides are
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Honeybees are facing an unparalleled threat from something that’s
causing them to leave their hives, never to return. Scientists call it
“colony collapse disorder” (CCD)  (Mystery of Disappearing Honeybees,
SiS 34). The major suspects in the murder of honeybees appear to be
systemic insecticides (the neonicotinoid systemic pesticides used
worldwide to treat seeds and crops), including genetically modified (GM)
crops [1, 2] (Requiem for the Honeybee, SiS 34), parasitic fungi 
(Parasitic Fungus and Honeybee Decline SiS 35), and radiation associated
with wireless phones  (Mobile Phones and Vanishing Bees, SiS 34).
It is unlikely, however, that the suspects act independently of one
another, and there is evidence suggesting that parasitic fungi and
pesticides interact synergistically in killing honeybees.
Parasitic fungi for biocontrol enhanced by sub-lethal levels of
Parasitic fungi are used extensively as biocontrol agents. Fungal spores
are applied in sprays or baits, and it has been observed that the
parasites frequently interact synergistically with neonicotinoid
pesticides, particularly imidacloprid, in killing insects. When the
spores are delivered as a suspension together with low, non-lethal
levels of the pesticide, the insect-killing activity of the fungal
spores is significantly enhanced. The spores of Beauveria bassinia used
to treat the brown leafhopper rice pest, when accompanied by a sublethal
dose of imidacloprid, killed the pest earlier and in larger numbers .
The fungus Lecanicillium muscarium in sublethal levels of imidacloprid
gave satisfactory control of the sweet potato whitefly, and merited
inclusion in integrated control programmes . Beauveria bassinia
spores combined with imidacloprid at a level one tenth the lethal dose
was found to significantly enhance control of the leaf cutting ant .
Similarly, termites were controlled by imidacloprid at sub-lethal levels
that enhanced the killing activity of the fungal parasite Metarhizium
anisopliae . The presence of the insecticides at sub-lethal level
appears to interfere with the insect’s immune system, making the insect
more susceptible to fungal pathogens.
Bees become exposed to sub-lethal levels of pesticide and biocontrol
The neonicotinoid insecticides used to dress seeds are systematic, and
accumulate in plant parts including the flowers. Hence honeybees
collecting pollen will become exposed to the pesticide, and become more
susceptible to fungal pathogens. The parasitic fungus, Nosema ceranae, a
single celled parasite was indeed found in CCD-affected bee hives from
around the USA .
Nosema locustae has been a commercial biocontrol fungus to control
locusts and grasshoppers. An integrated pest management strategy with an
emphasis on the use of Metarhizium, an ascosporic fungus, incorporates
low levels chemical pesticides with additional biological options such
as the microsporidian Nosema locustae and the hymenopteran egg
parasitoids Scelio spp. . Nosema bombycis has been a major pest of
the silkworm but it has been used to control Diamondback moth. Another
microporidian, Vairimorpha sp., isolated from the Diamondback moth in
Malaysia caused 100 percent mortality when applied to moth larvae at
1500 spores per larva . Nosema pyrausta infects the European corn
borer and can be used in biocontrol of the pest.
Parasitic fungi increases the killing power of Bt biopesticide
Evidence implicating Bt biopesticides from GM crops has also emerged.
Purified Bacillus thuringiensis Cry1Ab toxin was fed to Nosema infected
and uninfected borer larvae. Nosema infection reduced the lethal dose of
Cry1Ab toxin to one third the lethal dose of the uninfected larvae .
When Bacillus thuringiensis kurstaki (Dipel) formulations were used to
treat Nosema pyrausta infected and uninfected corn borer larvae. The
infected larvae had a lethal dose 45 times lower than the uninfected
I am not suggesting that biocontrol agents pose a threat to the
honeybee, rather, the exposure to sub-lethal levels of systemic
insecticides used in seed treatment of both conventional and GM crops
and in widespread soil and foliar applications can affect beneficial
insects by reducing their immunity to parasitic fungi. Furthermore, bees
that otherwise are unaffected by exposure to Bt toxins in GM crops may
succumb much more readily when they are infected with parasitic fungi,
as reported in an experiment carried out at the University of Jena,
Tests have been carried out on one agent at a time
Regulators have allowed extensive deployment of systemic insecticides
for seed treatment and they have allowed extensive use of foliar sprays
of the systemic insecticides on a wide array of food and feed crops. The
impact of such pesticides on honeybees has been evaluated using
measurements of lethal dose of the pesticides alone, ignoring the clear
evidence that sub-lethal doses of the insecticides act synergistically
with fungal parasites of the insects. The honeybees may be falling
victim to “friendly fire” directed to exterminating insect pests.
Unfortunately, regulators around the world have dealt with decline of
honeybees through tunnel vision, ignoring well-established
pesticide-fungal parasite interactions. It is time for the regulators to
wake up and impose a ban on the systemic pesticides before more bees
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